docs: def add_chain_ids(self, chains):
          """Add chains by ID into the chains attribute
  
          Args:
              chains (str, list): Chain ID or list of IDs
  
          """
          chains = ssbio.utils.force_list(chains)
  
          for c in chains:
              if self.chains.has_id(c):
                  log.debug('{}: chain already present'.format(c))
              else:
                  chain_prop = ChainProp(ident=c, pdb_parent=self.id)
                  self.chains.append(chain_prop)
                  log.debug('{}: added to chains list'.format(c))